This invention relates to a process for processing a lithographic printing plate using an aluminum plate as a support and utilizing the silver complex diffusion transfer process.
Some examples of a lithographic printing plate using the silver complex diffusion transfer process (the DTR process) are described on pages 101 to 130 of Andre Rott and Edith Weyde, "Photographic Silver Halide Diffusion Processes", published by the Focal Press, London and New York (1972).
As described therein, there have been known two kinds of lithographic printing plates utilizing the DTR process, i.e., a two sheet type in which a transfer material and an image-receiving material are separated and a mono-sheet type in which these materials are provided on one support. The two sheet type lithographic printing plate is described in detail in Japanese Provisional Patent Publication No. 158844/1982. Also, the mono-sheet type is described in detail in U.S. Pat. No. 3,728,114.
A mono-sheet type lithographic printing plate using an aluminum plate as a support and utilizing the silver complex diffusion transfer process (hereinafter referred to as "an aluminum lithographic printing plate"), which is the target of the present invention, is described in detail in Japanese Provisional Patent Publications No. 118244/1982, No. 158844/1982, No. 260491/1988, No. 116151/1991 and No. 282295/1992 and U.S. Pat. No. 4,567,131 and No. 5,427,889.
In the above aluminum lithographic printing plate, physical development nuclei are carried on a roughened and anodized aluminum support, and a silver halide emulsion layer is further provided thereon. A general process for making this lithographic printing plate comprises the steps of exposure, development processing, water washing processing (removal of a silver halide emulsion layer, hereinafter merely referred to as "washing processing") and finishing processing.
Specifically, metallic silver image portions are formed on physical development nuclei by development processing, and by washing processing subsequent thereto, a silver halide emulsion layer is removed to expose the metallic silver image portions (hereinafter referred to as "silver image portions") on an aluminum support. Simultaneously, the surface of anodized aluminum itself is exposed as non-image portions.
In a lithographic printing plate using the DTR process, silver halide crystals in which sensitivity specks are generated by exposure become blacked silver by chemical development. On the other hand, unsensitized silver halide crystals become a silver complex by a silver complexing agent in a developing solution, and the silver complex is diffused to physical development nuclei and causes a physical development in the presence of the nuclei to form silver image portions.
The aluminum lithographic printing plate which is the target of the present invention has physical development nuclei under a silver halide emulsion layer and therefore differs in constitution from a conventionally and generally used lithographic printing plate having physical development nuclei on a silver halide emulsion layer. It has been found that such difference in constitution between them exerts influence on the properties of a silver image formed by plate making processing.
That is, it is considered that transfer efficiency and the like may vary to exert influence on the manner of forming transferred silver depending on whether a silver complex in an emulsion layer is diffused upward (to a developing solution side) or downward (to a support side). Namely, the aluminum lithographic printing plate which is the target of the present invention has problems peculiar to an aluminum lithographic printing plate that transfer efficiency is poor, a silver image in which the amount of transferred silver is small is liable to be formed and silver image portions in which the amount of transferred silver is small, for example, thin lines and dots have bad inking property. These problems cause lowering of printing reproducibility, i.e., a problem that a silver image of a lithographic printing plate cannot be faithfully reproduced on a print.
Printing reproducibility refers to a characteristic that a silver image of a lithographic printing plate is securely inked and the same image as the silver image can be reproduced on a print. When printing reproducibility is good, there is an advantage that before printing, an image of a print can be estimated by observing a silver image of a lithographic printing plate, which is important when a lithographic printing plate is made. When printing reproducibility is bad, there is a problem that before printing, it is not sure whether a desired print can be obtained or not, which lowers the operation efficiencies of plate making and printing.
In general, the inking property of a silver image exhibits by imparting oil sensitivity to the silver image by making a compound which makes a silver image oleophilic (an oleophilic property-imparting agent), for example, a compound having a mercapto group or a thione group described in Japanese Patent Publication No. 29723/1973 and Japanese Provisional Patent Publication No. 127928/1983 act. Similarly, the inking property of the aluminum lithographic printing plate which is the target of the present invention is also improved by imparting oil sensitivity to silver image portions. However, it is difficult to impart oil sensitivity to a silver image formed with low transfer efficiency as described above so that the problem in printing reproducibility as described above is caused.
Also, in order to obtain high contrast and good ink receptivity in a printing region, it has been disclosed to carry out development in the presence of 1-phenyl-5-mercapto-tetrazole having a substituent such as a carboxy group in Japanese Provisional Patent Publication No. 72630/1995, and it has been disclosed to carry out processing in the presence of a benzotriazole compound in Japanese Provisional Patent Publication No. 319165/1995. However, desired properties cannot be obtained by the above techniques.
On the other hand, as a lithographic printing plate utilizing the DTR process, there have been known a camera type lithographic printing plate which is made by exposure for several seconds to several ten seconds by a process camera, and a lithographic printing plate for scanning type exposure, which is directly made by using the beam of a laser (a helium-neon laser, an argon laser, a semiconductor laser, a light-emitting diode or the like). Scanning type exposure has an advantage that sharpness and resolution are more excellent than those of exposure by camera.
However, there is a problem that even when a lithographic printing plate having high resolution can be obtained by scanning type exposure, if the inking property of a silver image is bad and thin lines and dots on a plate surface cannot be faithfully reproduced on a print as described above, it is impossible to make the best use of the advantage of the lithographic printing plate for scanning type exposure.